Method of and apparatus for the jet-pulverisation of fine grained and powdered solids

ABSTRACT

Fine-grained and powdered solids are jet-pulverised in a propellent-gas pipe by accelerating the particles within the jet by an at least substantially linear drop in pressure inside and along a jet tube and decelerating the mixture of propellent gas and solids on leaving the jet tube with an at least substantially linear increase in pressure and reducing the particles in size by impact.

United States Patent 11 1 1111 3,876,156 Muschelknautz et al. 1 Apr. 8, 1975 METHOD OF AND APPARATUS FOR THE 2.032.827 3/1936 Andrews 241/5 JET PULVERISATION OF FINE GRAINED 2332.407 /1943 Spenle 29/157 C 2.494.153 1/l950 Andrews et al. 241/5 AND POWDERED SOLIDS 2.560.807 7/1951 Lobo.... 241/1 [75] Inventors; Edgar Muschelknautz, Levcrkusen; 2.704.555 3/1955 Dali 138/44 Norbert Rink, Leichiingen, both f 2.768.938 10/1956 Mart1n.. 241/1 X Germany gefirzircsn C U 0.... [73] Assignee: Bayer Aktiengesellschaft, 3.482.786 12/1969 Hogg 241/40 X Leverkusen, Germany 3.523.350 8/1970 Ferri 29/157 C {22] Filed May 25 1972 3.643.875 2/1972 Dille 241/5 [2]] Appl. No.: 256,880 Primary E.\-aminerGranville Y. Custer, Jr.

Attorney, Agent, or FirmBurgess, Dinklage & Foreign Application Priority Data Sprung Dec. 29. 1971 Germany 2165340 [57] ABSTRACT [52] U.S. Cl 241/5; 241/40; 29/157 C Fine grained and powdered solids are jct pulverised in [51] int. Cl. B02c 19/06 a pmpe|1em gas pipe by accelerating the particles Field of Search 241/1, 5, 391 40; 138/44; within the jet by an at least substantially linear drop in 29/157 C pressure inside and along a jet tube and decelerating the mixture of propellent gas and solids on leaving the 1 1 References C'ted jet tube with an at least substantially linear increase in UNITED STATES PATENTS pressure and reducing the particles in size by impact.

238.044 2/1881 Luckenbach et al. 241/5 1.1147009 2/1932 Kollhohm 241 40 33 8 Drawmg F'gures E :1 3 E i H .r W

'3 1 diffusor length PATENTEDAPR B FIG. 8

METHOD OF AND APPARATUS FOR THE JET-PULVERISATION OF FINE GRAINED AND POWDERED SOLIDS This invention relates to a method of and apparatus for the jet-pulverisation of fine-grained and powdered. solids by means of a high-pressure jet which is formed by a propellent gas in which the solids are accelerated before being reduced in size by impact. The method is carried out in a machine consisting of a propellent-gas pipe with a material feeder opening into it, the propellent-gas pipe terminating in a jet tube which comprises an acceleration duct and which is adjoined by a diffusor, the orifice of the jet pipe being arranged opposite a baffle or impact surface.

For reducing fine-grained and powdered materials to extreme finenesses by impact, the solidparticles have to have impact speeds of at least 100 to several hundred metres per second. Purely mechanically, peripheral speeds and hence impact speeds of around 100 m/s are achieved in permanent operation with highstrength rotors, whilst peripheral speeds of around 200 m/s are obtained with contra-rotating rotors. Where higher impact speeds are required, the solid particles have to be accelerated in gas streams. v

Accordingly, air-jet or steam-jet impact pulverisers are used for grinding fine-grained and powdered solids to extreme fineness. In conventional spiral and ovaltube jet pulverisers, the grinding stock is delivered at low speed into the mill where it is picked up and accelerated by several gas jets. Speeds of up to 450 m/s are achieved where air is used as the propellent gas, whilst speeds of up to 1000 m/s are achieved where steam is used. However, due to the acceleration of the solid par:- ticles, the gas jets undergo such heavy deceleration that, at the usual stock throughputs of around 0.1 to 0.3 kg/h of solid per kg/h of gas, it is only possible to obtain low impact speeds of the solid particles of around 100 m/s in the case of steam. Impact speeds ofthis order are not always sufficient for ultra-fine grinding.

It is also known that, in so-called injector contra-jet mills, the solids are accelerated more effectively than in conventional spiral and oval-tube jet mills for throughputs of around 0.5 kg/h of solid per kg/h of gas. In this case, however, heavy losses through impact and friction occur in the injector tube so that the energy of the propellent-gas jet is only partly used for accelerating the product particles. Another disadvantage of the injector contra-jet mill is that extraneous air penetrates under suction into the stock feeder. This extraneous air has also to be accelerated so that jet energy is unnecessarily used. In the case of steam-jet mills, the extraneous air also gives rise to venting problems. Although it is possible to reduce the quantity of extraneous air by a correspondingly higher stock throughput, this gives rise to the formation of unstable conditions in the injector tube, resulting in a blow out of the mill. Injector jet mills undergo much heavier wear in the injector tubes than other conventional jet mills. v i

The object of the invention is to provide a method of and machines for carrying out the method in order to obtain for the purposes of ultra-fine grinding extremely high impact speeds with considerably higher stock throughputs and a much lower consumption of gas than in conventional jet mills.

According to the invention, this object is achieved in terms of process technology by virtue of the fact that acceleration in the high-pressure jet is carried out with an at least substantially linear pressure drop inside and along a jet tube and further by virtue of the fact that, on leaving the jet tube, the mixture of propellent gas and solids is decelerated with an at least substantially linear increase in pressure. In the context of the invention, the expressions substantially linear pressure drop and substantially linear increase in pressure" are intended to mean that the linearcurves for the drop in pressure and increase in pressure can show deviations of up to 25% and preferably of up to 15%.

The connections are explained in more detail in FIG. 1 of the drawing where the pressure is plotted against the length of the jet tube and the length of the diffusor. Both the jet tube and the diffusor are shown, the diffusor having been turned through in order more clearly to illustrate the distribution of pressure. The

pressure p prevails at the inlet end of the jet tube from which it falls linearly along the curve C to the value p at the end of the jet tube in accordance with the theory for obtaining optimum results. In practice, however, the drop in pressure generally follows the curve A or the curve Bi,- which deviate slightly from the curve C,,-. The deviation of curve A from curve C, amounts to A p, 0.25 (p 12 The deviation of curve B,- from curve C amounts to A 11 0.1 (p p The impact speed is hardly affected at all in the event of deviations A and B from the curve C Between the jet tube and the diffusor thereis a transition member in which thepressure p remains substantially constant. The pressure then increases linearly again in the diffusor itself from the value p to the value p along the curve C,, in accordance with theory. Curves A,, and B similar in shape to the curves A and B are encountered in practice. The deviation of curve A from curve C,, amounts to A p 0.l (p p Curve Bi, differs from curve C by the amount A p 0.25 (pg p Due to the linear pressure drop required in the jet tube, considerably greater tube lengthsare required than in the case of conventional jet mills. Due to the greater tube length, there are greater losses of energy attributableto wall friction so that hitherto experts have not followed the path according to the invention. Contrary to all expectations, however, it has been found in the process according to the invention that the overall losses are considerably lower than in conventional jet tubes.

In one preferred embodiment of the method according to the invention, two propellent-gas jets laden with solids are directed towards one another as known per se. In this way, the solids particles collide with one another so that a higher yield of energy is obtained than in cases where the solids particles collide with a fixed baffle surface. l I 1 Depending upon the impact velocities required, air, superheated steam, vapours of organic substances such as methylene chloride, chlorobenzene, dimethyl formamide or light gases, such as hydrogen and helium, are

accelerated to highvelocities with very little loss of energy. Accordingly the energy requirement is reduced by about half by comparison with conventional jet mills. The boundaries of the gas jet in the jet tube for producing a substantially linear drop in pressure and the boundaries in the diffusor for producing a substantially linear increase in pressure are essentially determined by the grain size of the solid particles introduced and by the required impact velocity.

One machine according to the invention for carrying out the method is distinguished by the fact that the jet tube consists of a cylindrical sleeve in which inserts are replaceably arranged in such a way that, looking in the direction of flow, they form together with one another over the length of the jet tube a flow duct which initially tapers conically and which thereafter extends cylindrically over a cettain distance and, finally, widens again.

The requisite duct cross sections of the jet tube over its length can readily be calculated for producing the required linear decrease in pressure, the most important factors being the grain size of the solid particles introduced and the required impact velocity. A jet tube of this kind has a length of approximately 0.3 to 3.0 metres. In the case of short tube lengths, the jet tube can still be made relatively profitably in one piece. In the case of fairly long jet tubes, mechanical production of the duct presents a few difficulties involving high costs on account of the necessary accuracies. For this reason, replaceable inserts are used. These inserts can be individually made. In this connection, it is even possible to accept minor deviations of the duct from its ideal form without any appreciable effect upon the optimum effectiveness of the jet tube. Accordingly, the replaceability of the inserts enables the jet tube to be adapted to meet various requirements such as, for example, change of the propellent gas, greater or lesser fineness of the grinding stock, or changes in the grain size of the solid particles introduced.

ln analternative embodiment of the machine for carrying out the method, the jet tube has a rectangular cross section whilst at least two opposite walls of the duct consist of a wear-resistant elastic material, being designed to be adjusted in sections towards the middle of the duct. In this way, not only can the flow duct be narrowed without any change in the contours, the contours can also be varied in order to be able to adapt the cross section along the duct to meet requirements.

In both embodiments of the machine, the widening of the duct is preferably continuous, in other words the jet tube widens purely conically or conically without any sudden jumps from the cylindrical section towards the outlet. In an alternative embodiment, the duct widens in stages. In other words, cylindrical sections of constant duct cross section can be provided in this case as well, providing for easier manufacture. It has been found that deviations from the ideal from beyond the cylindrical duct section are almost unnoticeable.

In one embodiment of the invention, the diffusor adjoining the jet tube is in the form of a plate diffusor. Alternatively, the diffusor can consist of at least two tubular diffusors arranged symetrically with respect to one another perpendicularly of the axis of the jet tube. The tubular diffusors preferably have a rectangular internal cross section. The use of plates or tubular diffusors, which is known per se, has the particular advantage that these designs are particularly suitable for obtaining the required increase in pressure in. the diffusor.

In one particular embodiment of the apparatus, an impact chamber provided with gas inlets and outlets is arranged opposite the orifice of the jet tube. By introducing or removing gas in the vicinity of the impact chamber, it is possible to obtain special effects in regard to the fineness of the grinding stock.

In another advantageous embodiment, two jet tubes are arranged coaxially directed towards one another in known manner. In this way, it is possible to obtain a particularly intense impact effect of the particles on one another.

Various embodiments of the machine according to the invention are illustrated purely diagrammatically in the accompanying drawing and described in more detail in the following. In the drawings:

FIG. 1 shows the diagram which has already been discussed in connection with the method according to the invention showing the dependence of the linear pressure curve upon the length of the tube or diffusor.

FIG. 2 shows a machine with one jet tube.

FIG. 3 shows a particular embodiment of the machine illustrated in FIG. 2 with an impact chamber.

FIG. 4 shows a machine with jet tubes directed oppositely to one another.

FIG. 5 is a longitudinal section through the jet tube shown in FIG. 2 on a larger scale.

FIG. 6 is a longitudinal section through an embodiment of the jet tubes shown in FIG. 4 on a larger scale.

FIG. 7 is a cross section through the jet tube shown in FIG. 6.

FIG. 8 is a cross section through a diffusor of the kind shown in FIG. 4.

In FIG. 2, propellent gas is delivered under pressure to a jet tube 2 by means of a propellent-gas pipe I. A pipe 3 branches off from the propellent-gas pipe 1, opening above the level of stock in a container 4 of a stock feeder 5. The container 4 comprises a conical pneumatically ventable base 6 which is supplied with propellent gas through a pipe 7 branching off from the pipe 3. In this way, the fine-grained stock in the container 4 is kept in a free-flowing state and introduced in measured quantities into the gas stream. The mixture of propellent gas and solids then enters and is accelerated in the jet tube 2 shown in more detail in FIG. 5, a linear pressure drop being produced from an inlet opening 8 of the jet tube 2 to its outlet opening 9. Arranged opposite the outlet opening 9 is an impact plate 10 against which the solid particles are thrown and thus size-reduced. The propellent gas and the pulverised stock flow off radially through the adjoining plate diffusor 11. The jet tube 2 (FIG. 5) consists of a sleeve 12 in which inserts 13 are arranged. These inserts 13 can be held in position by means of fixing screws 14. The inserts have coaxial bores 15 whose cross section changes from the inlet opening 8 to the outlet opening 9. The insert 13a nearest the inlet opening 8 has a heavily conically tapering duct 15a adjoined by an insert 1312 with a more gently tapering duct 15b. Another insert 13c has a duct with only a very slight conical taper. An insert 13d is provided with a duct 15d of constant cross section. The following insert 13a has a widening duct l5e. This is followed by a final insert 13f with another conically widening duct 15]. The diameters of adjacent inlets and outlets of the inserts 13 are consistent with one another so that there are no sudden changes in the diameter of the bores 15. A plate 16 of the diffusor 11 is fixed to the sleeve 12 at the outlet opening 9 of the jet tube 2. Counterplate 17 (FIG. 2) is flat. The diffusor plate 16 has a bead 18 which is designed in such a way that a substantially linear increase in pressure is obtained in the diffusor. The inserts 13 consist of metal, preferably a hard metal, or a ceramic material, i.e. of wear-resistant material. The surface of the impact plate is either grooved or smooth according to the product.

In FIG. 3, the diffusor 11 following the jet tube 2' is provided with a flat counterplate 17 which has an opening 19 leading into an impact chamber 20 opposite the outlet opening 9 of the jet tube 2'. The rear wall of impact chamber 20 forms a gas-permeable impact plate 10 behind which gas can be introduced or removed through a pipe 21.

In the machine shown in FIG. 4, compressed propellent gas is introduced through a propellent gas pipe 22 into a mixing chamber 23 to which the solid particles 24 to be ground are added in measured quantities from a container 25 by way of a rotary feeder 26. The mixture of propellent gas and solids then enters a jet tube 27 in which it is accelerated and radially discharged through a diffusor 28. A similar jet tube 27 is symetrically arranged coaxially with the axis of the jet tube 27. It is supplied with solid particles 24' from a supply container 25' the particles delivered by way of example by means of a metering screw 26' to a stream of propellent gas introduced through a ring pipe 22.

The jet tubes 27 and 27' are designed in accordance with the embodiment shown in FIGS. 6 and 7. A rectangular sleeve 29 is formed of two fixed side walls 30 and 30 whilst the two opposite side walls 31 and 31' consist of an elastic material and are designed to be adjusted in sections. Side walls 31 and 31 are adjusted by screws 32 which are adjustably mounted in the sleeve 29 and press against mouldings 33 and 33' in external contact with the elastic side walls 31 and 31. In FIG. 4, the diffusor 28 is surrounded by a housing 34 whose outlet opening 35 opens into a separation cyclone 36 which separates the propellent gas from the ground material. In cases where a high-grade propellent gas is used. it is recycled to the feed inlet.

The diffusor 28, as shown in FIG. 8 consists of two tubular diffusors arranged symetrically opposite one another perpendicularly of the axis of the jet tube. The two tubular diffusors 28' and 28" are identical in structure and consist of sleeves 37 and 37' in which inserts 38 and 38' are arranged. These inserts 38 and 38 are held in position by fixing screws 39 and 39'. The throughflow ducts 40 and 40' change their cross section in such a way that a substantially linear increase in pressure takes place therein from the inlet opening 41 or 41 to the outlet opening 42 or 42'. This means that, as in the case of the acceleration duct 15 in FIG. 5, the throughflow cross section is varied in this case as well over the length of the diffusor duct 40 or 40. The inserts 38 and 38' are replaceable in order to be able to adapt the machine more effectively to meet different requirements.

What we claim is:

I. A method of pulverizing material wherein a gas stream in which the material is dispersed is passed through a jet tube for acceleration of the gas stream and is thereafter conveyed through a diffusor for deceleration of the gas stream and pulverization is effected between the jet tube and the diffusor by impaction of the material, characterized in that the acceleration is accompanied by a substantially linear drop in pressure and said deceleration is accompanied by a substantially linear increase in pressure.

2. Method of claim 1, wherein .the" propellant gas stream is under a pressure'of 2 atmfi 3. Method of claim l, .whereinthe propellant gas stream is under a pressure of 8 "30'a'tm.

4. Method of claim l,wherein the-propellant gas stream is under a pressure of 2-30 atm.-

5. Method according to claim 1, wherein the diffusor is disposed symetrically and perpendicularly with respect to the jet tube and said stream strikes the wall of the diffusor for impact.

6. Method according to claim 1, wherein two opposed gas streams, are directed coaxially against each other from opposed jet tubes into a common diffusor disposed symetrically and perpendicularly with respect to the jet tubes.

7. Method of claim 1, wherein the gas stream comprises air, superheated steam, vapours of organic substances, or light gases.

8. Method of claim 1, wherein the gas stream comprises methylene chloride, chlorobenzene, or dimethyl formamide.

9. Method of claim 1, wherein the gas stream comprises hydrogen or helium.

10. Method according to claim 1, wherein the diffusor is disposed perpendicular to the jet tube.

11. An apparatus for jet pulverization by impact of fine grained and powdered solids comprising:

a. a jet tube comprising a tapered section having an inlet end of relatively large cross-sectional area and an outlet end of relatively small cross-sectional area, a widening section having an inlet end of relatively small cross-sectional area in communication with the outlet end of the tapered section for receiving material discharged from the outlet end of the tapered section, and an outlet end of relatively large cross-sectional area, for conveyance of a propellent gas laden with said solids,

b. a diffusor in communication with the outlet end of the widening section of the jet tube for receiving solid-laden propellent gas from the jet tube and the diffusion thereof,

c. means for causing impaction of the solids for the pulverization thereof between the jet tube and the diffusor,

d. the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and a substantially linear pressure increase, respectively.

12. An apparatus according to claim 11, the diffusor being disposed for conveying the solid-laden propellent gas radially outward from the outlet end of the widening section of the jet tube.

13. Apparatus according to claim 12, and an impact surface disposed at the outlet into the diffusor for impaction of the solids.

14. An apparatus according to claim 11, the length of the jet tube being 0.3 to 3.0 meters.

15. Apparatus according to claim 1 1, wherein the diffusor is disposed perpendicular to the jet tube.

l6..An apparatus according to claim 11, wherein the jet tube outlet into the diffusor is arranged opposite an impact surface, the jet tube comprising a cylindrical sleeve in which removable inserts are arranged to form an acceleration duct which tapers conically from the gas receiving end to a cylindricalsection, and widens from the cylindrical section towardsthe outlet. 17. An apparatus'according'to claim 16, wherein the widening of the acceleration duct is continuous.

- 18. An.apparatusraccording to-claim 16, wherein the acceleration duct'widens in stages.

19. An apparatus according to claim 11, wherein the jet tube outlet into the diffusor is arranged opposite an impact surface, the jet tube being of rectangular cross section and having at least two opposite walls which consist of a, wear-resistant elastic material, the elastic material being adapted to be adjusted in sections towards the middle of the duct to form an acceleration duct.

20. An apparatus according to claim 11, wherein the diffusor is in the form of a plate diffusor.

21. An apparatus according to claim 1 1, wherein the diffusor comprises at least two tubular diffusor sections which are arranged symetrically and perpendicularly with respect to the axis of the jet tube. I

22. An apparatus according to'claim 21, wherein the tubular diffusor sections have a rectangular internal cross section.

23. An apparatus according to claim 11, wherein an impact chamber with gas inlets and outlets is arranged opposite the outlet opening of the jet tube.

24. An apparatus according to claim 11, wherein two jet tubes are arranged coaxially and directed towards one another for introduction of solid laden propellant gas into the diffusor.

. 25. Apparatus according to claim 11, the jet tube being composed of a number of removable inserts.

26. Apparatus according to claim 25, the diffusor being composed of a number of removable inserts.

.27 Apparatus according to claim 11, the diffusor being composedof a number of removable inserts.

28. Apparatus according to claim 11, the jet tube cross-section being-adjustable so as to permit adjustment of the cross-section to provide said substantially linear pressure drop 29. Apparatus according to claim 28, th being of rectangular cross-section.

30. An apparatus according to claim 11, the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and a substantially linear pressure increase, respectively, at a propellant gas pressure of about 2 to lOO atms.

31. An apparatus according to claim 11, the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and a substantially linear pressure increase, respectively at a propellant gaspressure of about 8 to 30 atms.

32. An apparatus according to claim 11, the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and 'a substantially linear pressure increase, respectively, at a propellant gas pressure of about 2 to 30 atms.

33. A method of pulverizing material wherein a gas stream in which the material is dispersed is passed through a jet tube for acceleration of the gas stream and is thereafter conveyed through a diffusor for deceleration of the gas stream and pulverization is effected by impaction of the material after acceleration thereof in the jet tube characterized in that the acceleration is accompanied by a sybstantially linear drop in-pressure and said deceleration is accompanied by a substantially linear increase in pressure.

jet tube 

1. A METHOD OF PULVERIZING MATERIAL WHEREIN A GAS STREAM IN WHICH THE MATERIAL IS DISPERSED IS PASSED THROUGH A JET TUBE FOR ACCELERATION OF THE GAS STREAM AND IS THEREAFTER CONVEYED THROUGH A DIFFUSOR FOR DECELERATION OF THE GAS STREAM AND PULVERIZATION IS EFFECTED BETWEEN THE JET TUBE AND THE DIFFUSOR BY IMPACTION OF THE MATERIAL, CHARACTERIZED IN THAT THE ACCELERATION IS ACCOMPANIED BY A SUBSTANTIALLY LINEAR DROP IN PRESSURE AND SAID DECELERATION IS ACCOMPANIED BY A SUBSTANTIALLY LINEAR INCREASE IN PRESSURE.
 2. Method of claim 1, wherein the propellant gas stream is under a pressure of 2-100 atm.
 3. Method of claim 1, wherein the propellant gas stream is under a pressure of 8-30 atm.
 4. Method of claim 1, wherein the propellant gas stream is under a pressure of 2-30 atm.
 5. Method according to claim 1, wherein the diffusor is disposed symetrically and perpendicularly with respect to the jet tube and said stream strikes the wall of the diffusor for impact.
 6. Method according to claim 1, wherein two opposed gas streams, are directed coaxially against each other from opposed jet tubes into a common diffusor disposed symetrically and perpendicularly with respect to the jet tubes.
 7. Method of claim 1, wherein the gas stream comprises air, superheated steam, vapours of organic substances, or light gases.
 8. Method of claim 1, wherein the gas stream comprises methylene chloride, chlorobenzene, or dimethyl formamide.
 9. Method of claim 1, wherein the gas stream comprises hydrogen or helium.
 10. Method according to claim 1, wherein the diffusor is disposed perpendicular to the jet tube.
 11. An apparatus for jet pulverization by impact of fine grained and powdered solids comprising: a. a jet tube comprising a tapered section having an inlet end of relatively large cross-sectional area and an outlet end of relatively small cross-sectional area, a widening section having an inlet end of relatively small cross-sectional area in communication with the outlet end of the tapered section for receiving material discharged from the outlet end of the tapered section, and an outlet end of relatively large cross-sectional area, for conveyance of a propellent gas laden with said solids, b. a diffusor in communication with the outlet end of the widening section of the jet tube for receiving solid-laden propellent gas from the jet tube and the diffusion thereof, c. means for causing impaction of the solids for the pulverization thereof between the jet tube and the diffusor, d. the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and a substantially linear pressure increase, respectively.
 12. An apparatus according to claim 11, the diffusor being disposed for conveying the solid-laden propellent gas radially outward from the outlet end of the widening section of the jet tube.
 13. Apparatus according to claim 12, and an impact surface disposed at the outlet into the diffusor for impaction of the solids.
 14. An apparatus according to claim 11, the length of the jet tube being 0.3 to 3.0 meters.
 15. Apparatus according to claim 11, wherein the diffusor is disposed perpendicular to the jet tube.
 16. An apparatus according to claim 11, wherein the jet tube outlet into the diffusor is arranged opposite an impact surface, the jet tube comprising a cylindrical sleeve in which removable inserts are arranged to form an acceleration duct which tapers conically from the gas receiving end to a cylindrical section, and widens from the cylindrical section towards the outlet.
 17. An apparatus according to claim 16, wherein the widening of the acceleration duct is continuous.
 18. An apparatus according to claim 16, wherein the acceleration duct widens in stages.
 19. An apparatus according to claim 11, wherein the jet tube outlet into the diffusor is arranged opposite an impact surface, the jet tube being of rectangular cross section and having at least two opposite walls which consist of a wear-resistant elastic material, the elastic material being adapted to be adjusted in sections towards the middle of the duct to form an acceleration duct.
 20. An apparatus according to claim 11, wherein the diffusor is in the form of a plate diffusor.
 21. An apparatus according to claim 11, wherein the diffusor comprises at least two tubular diffusor sections which are arranged symetrically and perpendicularly with respect to the axis of the jet tube.
 22. An apparatus according to claim 21, wherein the tubular diffusor sections have a rectangular internal cross section.
 23. An apparatus according to claim 11, wherein an impact chamber with gas inlets and outlets is arranged opposite the outlet opening of the jet tube.
 24. An apparatus according to claim 11, wherein two jet tubes are arranged coaxially and directed towards one another for introduction of solid laden propellant gas into the diffusor.
 25. Apparatus according to claim 11, the jet tube being composed of a number of removable inserts.
 26. Apparatus according to claim 25, the diffusor being composed of a number of removable inserts.
 27. Apparatus according to claim 11, the diffusor being composed of a number of removable inserts.
 28. Apparatus according to claim 11, the jet tube cross-section being adjustable so as to permit adjustment of the cross-section to provide said substantially linear pressure drop
 29. Apparatus according to claim 28, the jet tube being of rectangular cross-section.
 30. An apparatus according to claim 11, the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and a substantially linear pressure increase, respectively, at a propellant gas pressure of about 2 to 100 atms.
 31. An apparatus according to claim 11, the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and a substantially linear pressure increase, respectively at a propellant gas pressure of about 8 to 30 atms.
 32. An apparatus according to claim 11, the jet tube and the diffusor being adapted to subject the gas passing through them to a substantially linear pressure drop and a substantially linear pressure increase, respectively, at a propellant gas pressure of about 2 to 30 atms.
 33. A method of pulverizing material wherein a gas stream in which the material is dispersed is passed through a jet tube for acceleration of the gas stream and is thereafter conveyed through a diffusor for deceleration of the gas stream and pulverization is effected by impaction of the material after acceleration thereof in the jet tube characterized in that the acceleration is accompanied by a sybstantially linear drop in pressure and said deceleration is accompanied by a substantially linear increase in pressure. 